Paul Gebhardt

Interface engineering in nanocarbon–Ta2O5 hybrid photocatalysts

Hybridizing inorganic nanomaterials with carbon nanotubes and graphene constitutes a powerful approach towards creating new functional materials for environmental and sustainable energy applications. Their superior performance originates from synergistic effects based on charge and energy transfer processes at the hybrids interfaces. However, only few studies have been devoted so far towards rationally designing these hybrids. In this work we demonstrate that engineering interfaces as well as the morphology of the functional inorganic compound can maximise the synergistic effects in hybrids, thus further enhancing the hybrids photocatalytic properties. Particularly, we have stimulated the growth of ultra-thin single-crystalline layers of tantalum (V) oxide (Ta2O5) with preferred orientation at substantially reduced crystallisation temperatures, by utilising the graphitic CNT surfaces as seed crystals through heterogeneous nucleation. The resulting hybrids possess outstanding activities for the evolution of hydrogen via sacrificial water splitting that are about 35 times higher than those of comparable materials such as tantalates. The additional improvements in this hybrid are attributed to the single-crystalline nature of the coating, which alleviates transport of electrons to the interface, as well as the formation of a Schottky-type junction between the metallic nanocarbon and the semiconducting metal oxide, which facilitates charge transfer and thus charge separation at the interface.

Ordered gyroidal tantalum oxide photocatalysts: eliminating diffusion limitations and tuning surface barriers

In this work we synthesized well-ordered, Ta2O5 films with a 3D-interconnected gyroid mesopore architecture with large pore sizes beyond 30 nm and extended crystalline domains through self-assembly of tailor-made triblock-terpolymers. This has effectively eliminated diffusion limitations inherent to previously reported mesoporous photocatalysts and resulted in superior hydrogen evolution with apparent quantum yields of up to 4.6% in the absence of any cocatalyst. We further show that the injection barrier at the solid-liquid interface constitutes a key criterion for photocatalytic performance and can be modified by the choice of the carbon template. This work highlights pore and surface engineering as a promising tool towards high-performance mesoporous catalysts and electrodes for various energy-related applications.

Growth mechanism and electrochemical properties of hierarchical hollow SnO2 microspheres with a “chestnut” morphology

Hierarchical hollow microspheres (HHMSs) constitute a very popular class of materials for use as drug-delivery carriers, photocatalysts and electrode materials in batteries, owing to their large, porous surface area and mechanical integrity during intercalation reactions. Here, we used a template- and additive-free hydrothermal route to prepare an unusually shaped SnO2 material that comprises a hollow spherical morphology with uniform diameters and very thin petal-like nano-sheets grown perpendicularly on the spheres surface, resembling a “chestnut cupule”. We thoroughly investigated the formation mechanism by 119Sn Mossbauer spectroscopy, powder X-ray diffraction and X-ray photoelectron spectroscopy. Key to this process is the ultrasonic pre-treatment of an aqueous SnCl2 solution, followed by Ostwald “inside-out” ripening upon hydrothermal processing. This unique morphology has greatly improved the storage capacity and cycling performance of SnO2 as an anode material for lithium and sodium ion batteries compared with conventional SnO2 materials.

Ice Nucleation Activity of Graphene and Graphene Oxides

Aerosols can act as cloud condensation nuclei and/or ice-nucleating particles (INPs), influencing cloud properties. In particular, INPs show a variety of different and complex mechanisms when interacting with water during the freezing process. To gain a fundamental understanding of the heterogeneous freezing mechanisms, studies with proxies for atmospheric INPs must be performed. Graphene and its derivatives offer suitable model systems for soot particles, which are ubiquitous aerosols in the atmosphere. In this work, we present an investigation of the ice nucleation activity (INA) of different types of graphene and graphene oxides. Immersion droplet freezing experiments as well as additional analytical analyses, such as X-ray photoelectron spectroscopy, Raman spectroscopy, and transmission electron microscopy, were performed. We show within a group of samples that a highly ordered graphene lattice (Raman G band intensity >50%) can support ice nucleation more effectively than a lowly ordered graphene lattice (Raman G band intensity <20%). Ammonia-functionalized graphene revealed the highest INA of all samples. Atmospheric ammonia is known to play a primary role in the formation of secondary particulate matter, forming ammonium-containing aerosols. The influence of functionalization on interactions between the particle interface and water molecules, as well as on hydrophobicity and agglomeration processes, is discussed.